An old friend (with potential benefits) of Ali's, Duncan has no idea that Alison -- or Vivian, as he knows her -- is dead. When he finally puts the pieces together, he realizes that he was one of the last people to see Alison alive before her disappearance.

"He's someone with a link to Alison who they weren't aware of," Lachlan told us when we visited him on set during filming in November. "The red coat is a major clue. Honestly, they're keeping everything under wraps. I was talking to Marlene [King], the writer, and she was just saying that they want everything to stay a mystery to the actors. All the questions that we're asking as characters, we're also asking as actors. We don't know what's going on."

Duncan was a pretty significant part of Alison's life the summer before her death, so when Aria discovers that Alison kept him secret from her friends, she's particularly surprised -- especially given the fact that Alison apparently told Duncan all about her, Hanna, Spencer, and Emily. "It's really cool to play a character that comes into this show that has some history," Lachlan says.

In this week's episode, Lachlan appears exclusively with Lucy, and while we were on set, they shot their scenes in a very intriguing set piece: a tiny, turbulent airplane.

"It's definitely interesting!" Lachlan laughs when we ask about shooting in the plane, which crew members shook as they worked, simulating choppy air. "I can't say I've ever done it before. It's the third time I've ever been in a plane that big, and it's definitely the safest I've felt. It's hot and it's cramped, but we're having fun. We're pretending it's a freezing cold night and it's like ninety degrees in that thing."

Whenever a new guy pops up in Rosewood, he's always got an agenda of his own, but Lachlan tells us that Duncan is more reliable than some of the previous men connected to Alison.

"I think Duncan's trustworthy at this point, but you never know with this show!" he says. "I think that Lucy's character, Aria, feels the same way, because this is a big deal for the girls, that there's this link to Alison's past. I'm a clue, in a way, and I've made it fairly obvious that I'm shocked by the fact that she's dead and that all these weird things are going on. I don't think he's the sinister type."

With some potential for Duncan to return next season, there's still a chance for him to have a dark side. "I don't hold anything in stone right now, that's for sure," Lachlan says.

For more about Lachlan, check out
our previous interview, in which he talked everything from "Hunger Games," to vegemite, to perfecting his American accent. We've got our eye on this rising TV star, particularly during pilot season (hello, he has CW written all over him!) and after tonight's "Pretty Little Liars," airing at 8 p.m. on ABC Family, so will you.